1. Field of the Invention
The present invention relates to a method and apparatus for driving a display of a computer system, such as a liquid crystal display, for example. More specifically, the present invention relates to a method and apparatus for driving a display with an improved appearance without increasing the available color gradations.
2. Background
In recent years, liquid crystal displays (liquid crystal display device, LCD) have been widely used in various kinds of personal computers (PC) such as desktop type PCs as well as notebook PCs. An image to be displayed on such a liquid crystal display is processed by a graphics controller of a host device composed of a PC or the like, and then displayed on the liquid crystal display. In this case, even if an operating system (OS) of the host supports 256 gradations per each color of R (red), G (green) and B (blue), for example, often only 64 gradations (0 to 63—represented by six bits of information per color) are actually supported in the liquid crystal display. Accordingly, in the display, it is necessary that the effective gradations per each color be multiplied by four (quadrupled). In order to accomplish this, known methods of Frame Rate Control (FRC) may be utilized for achieving multi-gradation by controlling a lighting time of each dot (each pixel).
FIGS. 25(a) to 25(e) are views for explaining conventional FRC for the simple example of multi-gradation between a 63rd gradation and a 62nd gradation. As shown in FIG. 25(a), with regard to the 63rd gradation, each dot is constantly displayed in the 63rd gradation. Similarly, also with regard to the 62nd gradation shown in FIG. 25(e), each dot is constantly displayed in the 62nd gradation. However, in the case of a 62.5th gradation shown in FIG. 25(c), which is an intermediate gradation, the 62.5th gradation is realized by a visual average of a displayed pattern alternating between two frames (a frame N and a frame N+1) in which each dot is displayed in the 63rd and 62nd gradations. Note that, in the liquid crystal display, the two frames constitute one cycle, and in the case of viewing a certain pixel, if a polarity of the pixel is positive (+) in the first frame, the polarity turns negative (−) in the next frame. The pixel is driven by alternating current (for example, of 60 Hz) in a cycle of two frames. There are various methods for inverting the polarities of the liquid crystal pixels adjacent to one another depending on a type of the liquid crystal display. FIG. 25(c) shows a check pattern (a pattern where squares are arrayed in a staggered manner, as in a chessboard).
Moreover, in a 62.75th gradation shown in FIG. 25(b), the 63rd gradation and the 62nd gradation in each dot is mixed in a ratio of 2:1, and three different frames are prepared. Then, the 62.75th gradation is realized by a visual average created by alternatively displaying each of the three frames (the frame N, the frame N+1 and a frame N+2). Furthermore, in a 62.25th gradation shown in FIG. 25(d), the 63rd gradation and the 62nd gradation in each dot is mixed in a ratio of 1:2, and three different frames are prepared. Then, the 62.25th gradation is realized by a visual average created by alternatively displaying each of the three frames. Note that the ratio used is not a ratio of 1:3 or 3:1 but a ratio of 1:2 or 2:1 is employed—this is because a known visual element for a slew rate of the liquid crystal display, and the like, is considered.
There exists a well-known technology of changing a frame frequency depending on whether the number of colors in data to be displayed is a predetermined number of colors or less or exceeds the predetermined number of colors (for example, see Japanese Unexamined, Published Patent Publication No. 2002-149118, pp. 5-6, FIG. 1) in order to prevent flickering on a color liquid crystal screen mounted on a portable information terminal device.
Herein, there are several LCD driving methods well-known to those skilled in the relevant arts. In terms of inversion drive at a vertical line (V line) and a horizontal line (H line), there are 1H1V inversion LCD drives, 2H1V inversion drives, 1H2V inversion drives, 2H2V inversion drives, and the like. The 1H1V inversion LCD drive inverts the lines to form a normal check pattern. The 2H1V inversion drive inverts a pattern, which is inverted by every two H lines, for each V line. The 1H2V inversion drive inverts a pattern, which is inverted by each H line, by every two V lines. The 2H2V inversion drive inverts a pattern, which is inverted by every two H lines, by every two V lines.
Meanwhile, there are several known FRC methods for multi-gradation. Since the foregoing LCD driving methods are not considered in the conventional FRC methods which have been widely used, a fixed pattern display error occurs when an x.5-th gradation, for example, the 62.5th gradation is displayed (x is an integer more than or equal to 0 determined by gradations of a display, e.g., 0 to 62). In other words, for example, in the case where a 64 gradation LCD employs an FRC for 256 gradation display, a fixed pattern display error occurs when an image is displayed using conventional FRC methods—the type of error depending on a combination of the LCD driving method and the FRC pattern utilized.
When a mixture ratio of an A-th gradation to a B-th gradation, such as of the 63rd gradation to the 62nd gradation, is not 1:1, for example, when the mixture ratio is 1:2 or 2:1, there are three fixed patterns as shown in FIGS. 25(b) and 25(d). The alignment of these three fixed patterns is shifted in one direction. As a result, when a conventional FRC method is employed, a dynamic display error (wave stripe) occurs, in which an oblique stripe pattern is seen as flowing across the display.
In the Published Japanese Patent Application listed above, flickering is prevented by changing a frame frequency. However, this technique can effectively be applied only when LCD resolution is low as in mobile phones (e.g., 240×320 dots) and there is the possibility of increasing the screen frequency. For example, the screen resolution of a PC display of the Extended Graphics Array (XGA) type may be 1024×768 dots This resolution is approximately 10 times larger than that of the mobile phone. Moreover, a pixel transfer rate is approximately 100 MHz by two pixel simultaneous transmission in PCs, and it is almost impossible to maintain a screen frequency at 60 Hz, where a flicker is invisible. Accordingly, it is difficult to increase the screen frequency. In addition, a screen frequency of a high resolution LCD is unlikely to be increased since the increase will lead to increases in both power consumption and manufacturing costs. Therefore, it is impossible to apply the technique disclosed in the Published Japanese Patent Application discussed above to an LCD for a PC or the like.